Automated mechanical transmission systems employing microprocessor based electronic control units which respond to various vehicle operating condition or to operator inputs to effect a gear ratio change or shift are well known in the art. See, for example, U.S. Pat. No. 5,053,962 assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference. An electric motor driven shift mechanism may be used to perform the shift operation in response to either a manual or automatic shift initiation. See, for example, U.S. Pat. No. 4,873,881 assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference. In U.S. Pat. No. 4,873,881, a shift finger is driven by one or more DC motors along an X--X axis to select a particular shift rail and along a Y--Y axis to effect movement of a sliding clutch into or out of engagement with a gear.
There are three possible movements involved in the shift of an automated mechanical transmission. The three movements are (a) pull to neutral, (b) rail change and (c) gear engagement. During pull to neutral the sliding clutch of the transmission disengages from one of the transmission gears and moves to a neutral or out of gear position. During gear engagement the sliding clutch is moved from the neutral position into gear engagement with the selected gear. It is important that both gear disengagement and gear engagement be as smooth as possible to maximize shift quality and minimize cab lurch or jerk. Cab lurch occurs when an unintended or unexpected gear engagement or disengagement occurs.
During a pull to neutral operation, the sliding clutch is preloaded by the shifting mechanism with a predetermined force. This preload force is less than the frictional force normally developed by driveline torque acting on the clutch and the clutch therefore remains engaged with the gear. However, oscillations in the driveline may cause the frictional sliding force developed at the sliding clutch by the driveline to drop below the preload force. This can result in an premature gear disengagement causing cab lurch. It is preferable that disengagement take place during the zero torque shift window which occurs normally during deceleration of the engine relative to the vehicle. Also, if disengagement is not accomplished during the zero torque window, the driveline torque will reverse direction causing the shift yoke and subsequently the electric motor to rapidly stall. Because the motor has substantial inertia, this stall condition may result in sufficient force on the shifting mechanism to cause the sliding clutch to "rattle" out of gear, producing both cab lurch and substantial impact damage to the mechanical linkage.
During engagement of the clutch with a gear, the force applied should be only slightly greater than the frictional force developed on the sliding clutch by the driveline. Even though the controller will not attempt to engage the sliding clutch with the gear until the speed of the two are nearly synchronous, out of synchronous engagement may occur which will cause cab lurch and damage to the sliding clutch unless through control of the force being applied to the sliding clutch, this out of synchronous engagement can be negated.